The invention relates to an electrical-discharge electrode, in particular spark-plug electrodes, having a main electrode body made of metal and a method of producing it.
Contemporary spark plugs have, as a rule, a central electrode and an earth electrode, the tips of the two electrodes being arranged with respect to one another in such a way that a spark gap is left free. The tips are subject to an appreciable wear as a result of the continuous spark production between the two electrodes. This problem imposes strict requirements on the high-temperature resistance, corrosion resistance and thermal expansion characteristics of the electrode tip. Spark erosion and oxidation phenomena also result in an appreciable stress.
In order to improve, in particular, the electrode tip, U.S. Patent Specification 4 540 910 therefore proposes, for example, that an interlayer which serves to compensate for the markedly different thermal expansion behaviour of electrodes and metal coatings is disposed between the metal electrode, which is composed of a nickel alloy, and a metal layer with high wear resistance, which is made of a platinum-containing alloy. This interlayer is composed of an alloy which is made up of a platinum alloy and nickel. To apply the wear-resistant metal layer to the metal electrode, the wear-resistant metal layer is first mechanically bonded to the interlayer and then the interlayer provided with the metal layer is joined to the metal electrode by resistance welding. The production expenditure necessary for this purpose and, in particular, the high material costs are appreciable, and furthermore, only the thermal expansion behaviour is improved.
German Patent Specification 31 32 814 furthermore discloses that a platelet made of noble metal, such as, for example, platinum, is applied by resistance welding to the free end face of a central electrode of a sparking plug. In the case of this central electrode, however, the problem then arises that the noble-metal platelet peels away from the central electrode as a result of stresses in the joint zone under fairly high thermal and corrosive loads. Here, again, even the material costs are appreciable.